Biomass Characteristics - Biomass Gasification, Pyrolysis and Torrefaction - page 58

Cellulose is highly insoluble and, though a carbohydrate, is not digestible by
humans. It is a dominant component of wood, making up about 40

44% by dry
weight. Cellulose is a major contributor of tar during gasification of biomass.

3.3.2.2 Hemicellulose
Hemicellulose is another constituent of the cell walls of a plant. While cellu-
lose has a crystalline, strong structure that is resistant to hydrolysis, hemicel-
lulose has a random, amorphous structure with little strength ( Figure 3.8 ). It
is a group of carbohydrates with a branched chain structure and a lower
degree of polymerization (DP 200) and may be represented by the

generic formula (C 5 H 8 O 4 ) n (Klass, 1998, p. 84). Figure 3.8 shows the molec-
ular arrangement of a typical hemicellulose molecule, xylan.
There is significant variation in the composition and structure of hemicel-
lulose among different biomass. Most hemicelluloses, however, contain
some simple sugar residues like D -xylose (the most common), D -glucose,
D -galactose, L -ababinose, D -glucurnoic acid, and D -mannose. These typically
contain 50

, 100

200 units in their branched structures.
Hemicellulose tends to yield more gases and less tar than cellulose
(Milne, 2002). It is soluble in weak alkaline solutions and is easily hydro-
lyzed by dilute acid or base. It constitutes about 20

30% of the dry weight

of most wood.
The hemicellulose content of a hardwood and a softwood sample could
be comparable, but their behavior during torrefaction could be very different
because of the variation in the composition of the hemicellulose of these
samples. Main constituents of softwood hemicellulose are galactoglucoman-
nans and arabino-glucuronoxylan ( Table 3.4 ). In case of hardwood, it is glu-
curonoxylan, also called xylan. As one can see in Table 3.4 , the glucoronox-
ylan (DP 200) in hardwood is in the range of 10 35%, while that in the

form of arabino-glucuronoxylan (DP 100) is in the range of only 7

15%.

3.3.2.3 Lignin
Lignin is a complex highly branched polymer of phenylpropane and is an
integral part of the secondary cell wall of plants. It is primarily a three-
dimensional polymer of 4-propenyl phenol, 4-propenyl-2-methoxy phenol,
and 4-propenyl-2.5-dimethoxyl phenol (Diebold and Bridgwater, 1997)

OH OH

O OO

OH OH

OH

OO OO

OH

FIGURE 3.8 Molecular structure of a typical hemicellulose, xylan.